The present invention relates to an electrophotographic image forming apparatus having an image carrier for forming an electrostatic latent image thereon and, more particularly, to a charging device for charging the image carrier.
Electrophotographic copiers, laser printers and facsimile machines belong to a family of electrophotographic image forming apparatuses having an image carrier in the form of a photoconductive drum or a photoconductive belt. Electrophotographic methods for this type of image forming apparatus include an indirect electrophotographic method which charges the surface of the image carrier uniformly, exposes the charged surface to image data, e.g., a reflection from an original document to form an electrostatic latent image thereon, develops the latent image by a toner or similar developer, transfers the resulting toner image to a plain paper or similar recording medium, and then fixes the toner image on the medium by heat and pressure. A direct electrophotographic method is another conventional electrophotographic method and uses a recording medium itself as an image carrier. This kind of method charges the surface of the medium uniformly and then sequentially executes the exposing, developing and fixing steps with the medium. In any case, the electrophotographic method charges the surface of the image carrier uniformly at the beginning of image formation.
To charge the surface of the image carrier uniformly, as stated above, various kinds of charging devices are available and may generally be classified into devices using corona discharge, devices using a brush, and devices using a roller. A corona discharge type charging device deposits a charge on the surface of the image carrier with one or more wires for corona discharge. Specifically, this type of device has a shield having an opening facing the image carrier, and one or more tungsten wires or gold-plated tungsten wires disposed in the shield. A high voltage of 4 kV to 7 kV in absolute value is applied to the wires to effect corona discharge. Among this type of charging devices, a scorotron charger is provided with a grid electrode between the wires and the image carrier in order to promote uniform and stable charging.
On the other hand, a brush type charging device has a conductive brush connected to a power source and is made of metal or conductive resin. The brush is held in contact with the image carrier for charging the surface of the image carrier. This type of device differs from the corona discharge type device in that it is operable with a relatively low voltage which is substantially the same in potential as a target charge level. A roller type charging device uses a roller consisting of a metallic shaft and one or more layers of conductive rubber covering the shaft. This type of device applies a voltage to the roller while pressing it against the image carrier. Such a charging device, Nike the brush type charging device, can operate with a relatively low voltage and, in addition, produces only a small amount of ozone.
All the conventional charging devices, however, have some issues yet to be solved, as follows. To begin with, the corona discharge type device needs a voltage as high as 4 kV to 7 kV in absolute value. Hence, the wiring for the device has to be connected and distinguished from the other wirings with greatest care. Moreover, corona discharge produces ozone. Particularly, negative corona discharge produces more than ten times the amount of ozone than positive corona discharge. Such an amount of ozone limits materials available for the parts built in the image forming apparatus as well as reliability of operation. Further, to prevent ozone from leaking to the outside, an ozone filter is needed and has to be replaced often, increasing the running cost of the apparatus. In addition, products deposited on the wire surfaces due to corona discharge degrade the discharging ability and, therefore, reliability of the discharging device itself.
Although the brush type and the roller type discharging devices produce a minimum amount of ozone, they are apt to scratch the surface of the image carrier since the former contacts the latter. Further, the conductive brush for example, is smeared due to defective cleaning of the image carrier and the entry of developer and paper dust in the charging device, resulting in the fall of charge potential. Irregular charging is also brought about by irregularities particular to production and assembly lines. Particularly, the brush type charging device has various problems relating to the density of the brush, the fall-out of bristles, and the conditions for the contact of the brush with the image carrier. Although a charging device using a multi-stage brush scheme has been proposed, it is also problematic with respect to cost and space. The roller type charging device can obviate many of the problems of the brush type charging device. This, coupled with the fact that rollers of uniform configuration can be produced relatively easily and can be uniformly pressed against the image carrier, has put the roller type device to practical use. However, once the surface of the roller is scratched or otherwise disfigured, image quality is lowered since the disfigured portion differs in charging ability from the other portion. Also, this type of charging device is questionable as to whether or not it can implement further uniform charging matching the increasing image density. In addition, such a device is not applicable to a multicolor developing process which forms color images one above the other on the image carrier.
U.S. Pat. No. 4,819,028 and Japanese Patent Publication No. 63-43749 respectively disclose a specific form of the brush type charging device. Further, Shunji Nakamura et al. teach a specific form of the roller type charging device in a paper entitled "THE MECHANISM OF CHARGING ROLLER".